1. Field of the Invention
The present invention relates generally to a device and a method for adjusting a loop filter gain in an automatic frequency controller, and in particular, to a device and a method for adjusting a loop filter gain in an automatic frequency controller, which can allow the automatic frequency controller to operate while maintaining optimal performance.
2. Description of the Related Art
Since a carrier frequency depending on temperature gradually changes, a frequency offset is an unavoidable primary factor to cause a deterioration of performance in a radio communication system. If there exists a frequency offset between a carrier frequency and a local oscillation frequency of a receiver, a power spectrum density of a channel observed from the receiver can be expressed by the following formula.
      S    ⁡          (      f      )        =            b      0                      π        ·                  f          d                    ⁢                        1          -                                                    (                                  f                  -                                      f                    offset                                                  )                            2                                      f              d                                          Wherein bo is a contant, fd=Doppler frequency, and foffset=frequency offset.
This means that a change of a channel including a frequency offset is determined by means of an offset interval (foffset+fd). If other fading compensation techniques can also compensate a change in phase due to the frequency offset, an Automatic Frequency Controller (AFC) is omitted so that simplification of a receiver can be achieved. However, in most cases, the AFC for minimizing the frequency offset is required.
A Common Pilot Channel (CPICH) of a first antenna and a diversity CPICH of a second antenna become reference signals of a frequency control loop in a Wideband Code Division Multiple Access (WCDMA) system. At a reception terminal, frequency error components are shown in signals of the two CPICHs. A phase of the CPICH can be calculated as a mean phase by adopting an arbitrary time interval (Td) regardless of the transmission rate of a traffic channel. Integrating and dumping the reception signal at an interval (t−Td/2, t+Td/2) can provide for the phase of a reception signal. Since a pilot channel transmits an unmodulated signal, the AFC can calculate a change in phase by taking a cross product of a continuous pilot signal. The aforementioned result becomes a linear estimation value of the change in phase with respect to a small phase change, and the change in phase is precisely in proportion to a frequency error.
The change in phase is generated because a terminal has a relative timing inaccuracy with respect to a base station. A timing reference of the terminal is a Temperature Crystal Oscillator (TCXO), and the timing inaccuracy is generated due to a small frequency error of the TCXO. Thus, a frequency of the TCXO is adjusted to set timing between the terminal and the base station.
A change in phase of the CPICH is accumulated in the AFC, and the AFC determines whether its sign is much faster than the frequency of the TCXO so as to compensate for a frequency error.
The AFC is configured within a modem provided in a terminal. The AFC has a frequency error detector detecting a frequency error, a loop filter infinitely accumulating the detected error and a Pulse Density Modulation (PDM) unit. The PDM unit functions as a Digital Analog Converter (DAC) for converting an output of the loop filter into an analog voltage to control an output frequency of a VCTCXO (Voltage Controlled Temperature Crystal Oscillator).
An operation of the AFC is divided into an acquisition step and a tracking step. In the acquisition step, since a frequency should be within a certain frequency error range in a rapid time, the bandwidth of a frequency control loop becomes broad. On the other hand, to reduce an influence cause by a noise in the tracking step, the bandwidth of a frequency control loop becomes narrow to reduce residual frequency error.
Each of the acquisition and tracking steps has a predetermined loop filter gain K to adjust a bandwidth. After a certain time, the acquisition step moves to the tracking step, and the tracking step has a predetermined small loop filter gain K to use a narrow bandwidth. There is a high possibility that a user equipment (UE) which accelerates in a short time period, such as on a rapid transit railway will lose a lock state due to the bandwidth of a small loop in the tracking step. Further, a Quadrature Amplitude Modulation (QAM) method is used in a High Speed Downlink Packet Access (HSDPA) for a high speed packet data service in the future, and the QAM modulation method is more sensitive for a frequency error. Therefore, there is a disadvantage in that the conventional AFC having a fixed loop bandwidth through a fixed loop gain is not flexible for various circumstances.